Conventional kicker magnets used in circular accelerators, such as synchrotrons, are designed as several circuit elements constituting distributed constant circuits (for example, see non-patent document 1). Here, the kicker magnets are electromagnets used when charged particle beams are injected to circular accelerator rings, or the charged particle beams are ejected from the circular accelerator rings.
FIG. 1 is a diagram schematically illustrating a circular accelerator including conventional kicker magnets. As shown in FIG. 1, in a circular accelerator 10 such as a synchrotron, several bending magnets 11, arranged in a ring, generate magnetic fields for bending a charged particle beam 12. Furthermore, a radio frequency accelerator 13, arrange in a part of the ring, circumferentially generates a radio frequency accelerating electric field. With this, the charged particle beam 12 accelerates as it circulates around an orbital path repeatedly. At this time, the radio frequency accelerating electric field also acts as a restoring force. Thus, the charged particle beam 12 localizes and circulates along the ring of the circular accelerator 10 while forming a cluster called a bunch. After sufficient acceleration, the charged particle beam localizing and circulating along the ring, that is, a bunched charged particle beam (hereinafter, referred to as a beam bunch) is ejected from the ring with a fast beam extraction method and the like.
In the fast beam extraction method, the kicker magnet 15 for ejection is excited at high speed in order to eject the beam bunch 12 from the ring. At this time, the kicker magnet 15 for ejection generates a magnetic field which rises at around 50 to 200 nsec, and has a magnetic field strength of around 20 to 50 mT. This is because the kicker magnet 15 for ejection needs to have fast response characteristics and to generate a magnetic field having a sufficient strength, in order to bend the beam bunch 12 which is in a high energy state made by being accelerated by the circular accelerator 10.
FIG. 2 is a diagram schematically illustrating a fast electromagnet device including a conventional kicker magnet. As shown in FIG. 2, a fast electromagnet device 14 excites the kicker magnet 15 for ejection, provided at the straight section of the ring of the circular accelerator 10, at high speed. Here, as an example, the fast electromagnet device 14 includes a pulse power supply 21, a coaxial cable 22, the kicker magnet 15 for ejection (hereinafter, referred to as kicker magnet 15), a terminating resistor 23, and the like. The output terminal of the pulse power supply 21 and the input terminal of the kicker magnet 15 are connected each other through the coaxial cable 22. The output terminal of the kicker magnet 15 is connected to one end of the terminating resistor 23. The other end of the terminating resistor 23 is grounded.
Further, the pulse power supply 21 includes a DC charge power supply, Pulse Forming Network (PFN), a thyratron, and the like. In order to excite the kicker magnet 15, the thyratron is fired after the PFN is charged in advance by the DC charge power supply, so that a high voltage is outputted. This causes a high voltage having a pulse waveform (hereinafter, referred to as pulsed voltage) to be applied to the kicker magnet 15 from the pulse power supply 21 through the coaxial cable 22 which is one kind of transmission lines. As a result, as shown in a waveform 33, a current having a pulse waveform (hereinafter, referred to as pulsed current) flows, thereby driving the kicker magnet 15.
At this time, it is necessary for the fast electromagnet device 14 to start excitation of the kicker magnet 15 immediately after a predetermined beam bunch passes through the kicker magnet 15, complete the excitation of the kicker magnet 15 before the next beam bunch reaches the kicker magnet 15, and generate a predetermined magnetic field. In other words, the time period t which is necessary for generating the magnetic field at the kicker magnet 15, needs to be designed to be less than the time difference between the beam bunches.
In general, the number of the beam bunches existing around the ring of the circular accelerator 10 and the time difference between the beam bunches, are determined by the design or driving parameter of the circular accelerator 10. Typically, the number of the beam bunches is in the range from one to several thousand, and the time difference between the beam bunches is in the rage from several dozen to several hundred nsec. Therefore, as shown in a waveform 32, as a rise time for excitation of the kicker magnet 15, a response characteristic approximately ranging from several dozen to several hundred nsec is required. Here, in a waveform 31, the horizontal axis represents time, and the vertical axis represents intensity of beam bunch measured at the position where the kicker magnet 15 is provided. Further, in the waveform 32, the horizontal axis represents time, and the vertical axis represents strength of the magnetic field of the kicker magnet 15.
As described, since the fast electromagnet device 14 is required to have fast response characteristics, it is necessary to prevent reflection of current flowing through the kicker magnet 15 from occurring. Thus, it is necessary to match the input impedance of the kicker magnet 15 and the characteristic impedance of the coaxial cable 22 such that they become equal to each other. However, the characteristic impedance of the coaxial cable 22 is usually treated as a pure resistance, and is independent of frequency. On the other hand, it is assumed that the kicker magnet 15 is designed as one of the circuit elements, such as a coil, constituting a lumped constant circuit. In this case, the input impedance of the kicker magnet 15 becomes a function of frequency, and cannot be matched with the characteristic impedance of the coaxial cable 22. Thus, the kicker magnet 15 is designed as several circuit elements constituting a distributed constant circuit
FIG. 3 is a diagram schematically illustrating a conventional kicker magnet. As shown in FIG. 3, as an example, the kicker magnet 15 includes several units each having a magnetic core 15d, and electrode plates 15a, 15b, and 15c which sandwich the magnetic core 15d, and is designed as circuit elements 16a, 16b, and 16d constituting the distributed constant circuit. This results in making the input impedance of the kicker magnet 15 constant only in the band equal to or lower than a predetermined cutoff frequency, without depending on frequency. Then, it is possible to perform a matching with respect to a major component among high-frequency component of the current flowing through the kicker magnet 15.
Non Patent Document 1: KEK-76-21, K. Takata, S. Tazawa, and Y. Kimura, “FULL APERTURE KICKER MAGNETS FOR KEK PROTON SYNCHROTRON.” (1977).